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Homework Help: Kinetic Energy/work finding the distance

  1. Feb 13, 2012 #1
    A 12 kg block is sliding to the left across the floor at a speed of 18 m/s. Opposing the
    motion is a 50 N force. How far (in m) will the object travel before its speed is reduced
    to 8 m/s?
    (a) 21.4
    (b) 31.2
    (c) 18.4
    (d) 7.7
    (e) none of these

    2. Relevant equations[/b]

    lost on how to find distance tho..

    K.E. = 1/2mv^2

    3. The attempt at a solution
    I got d but not sure...i thouhght it was d 7.68 round up
    Last edited by a moderator: Feb 15, 2012
  2. jcsd
  3. Feb 13, 2012 #2
    Re: finding the distance using kinetic energy

    You should be able to use the Work-Energy Theorem to solve this one.


    You know the change in the block's kinetic energy, and you know that friction does negative work. It does not say that there is any other force pushing the block along, so friction is the only force doing work on the block.

    Since work is force x distance, this becomes a simple "plug and chug" problem.
  4. Feb 13, 2012 #3


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    Staff Emeritus
    Science Advisor
    Gold Member

    Use the work-energy theorem. Since you know the change in kinetic energy, you can find the work done by the 50 N force, and from that, you can find the distance over which that force was applied.
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